The invention is related to free-space optical communications systems and may be used for two-way information transfer between remote objects without use of wires and/or optical fibers, including the case when multiple users are taking part in the information exchange, e.g. if network (mesh) technologies are used for information transfer, or if point-to-multipoint technology is implemented for a two-way information exchange between a base station and several subscribers.
The modern information age has developed a pattern of continually increasing demand for communication capacity. A variety of optical fiber and wired technologies now provide high bandwidth with attendant high data rates for communications to customer premises, but installation of such facilities is not practical in all locations. In many major population centers, installation of a new physical facility of this type requires underground installation with a high construction cost. The construction and the attendant requirement for local government approval impose considerable time delays. In many instances, the actual available capacity through long delayed deployments lags far behind the ever-increasing demand. Radio frequency (RF) wireless solutions reduce the time, complexity and cost of installation, but those solutions are inherently limited by their use of shared RF spectrum. As the number of users on a given piece of spectrum grows, the average capacity available to any one user declines.
Free-space optical communications systems offer two-way information transfer between remote objects without use of wires and/or optical fibers. Because such systems can implement point to point links to the individual customer premises, such systems are not subject to the limits of shared capacity, as in the existing RF wireless technologies. Free-space optical communications systems may implement network (mesh) technologies for information transfer or point-to-multipoint technology for a two-way information exchange.
U.S. Pat. No. 5,786,923, for example, discloses a point-to-multipoint bi-directional wide area telecommunications network employing atmospheric optical communication. The network comprises a primary transceiver unit, an optical router, and subscriber transceiver units. The primary transceiver unit generates a first light beam, which carries modulated data intended for end users. The optical router receives the first light beam and demodulates the data. The optical router then remodulates the data onto a second light beam and transmits the second light beam to the subscriber transceiver units. The optical router demodulates and retransmits to each of the subscriber transceiver units in a time-multiplexed fashion. In the patented system, each subscriber transceiver unit includes an optical receiver/transmitter. This optical xe2x80x9cantennaxe2x80x9d is coupled to an input/output device, such as a set-top box and television set or a computer or television, by an optical fiber or cable. The subscriber transceiver units receive the second light beam and demodulate the data. In the upstream direction, each subscriber transceiver unit atmospherically transmits a third light beam, which includes data from the user equipment. The subscriber transceiver sends this beam to the optical router. The optical router demodulates the upstream data, modulates it onto a fourth light beam, and transmits the fourth light beam to the primary transceiver unit.
Free-space optical communications involve multibeam communications. Accordingly, key components of free-space optical communication systems are multibeam optical transmitters with beams that may be targeted at prescribed objects arbitrarily positioned in space, as well as optical receivers for reception of signals arriving from such objects. These components enable wireless optical data communications and associated networking.
Realization of multibeam optical transmitters or receivers using the multichannel principle, i.e. as a set of separate channels of the same type operating point-to-point, multiplies the weight, dimensions, and cost of the entire system at least N-fold if N beams or directions should be implemented for communications.
The basic parameter of a multibeam communication system is a bit rate or bandwidth allocated to a communication network subscriber. It is evident that in a prior art multichannel system ratios between bandwidth dedicated to a single subscriber and total weight and cost of the entire system, decreases at least as fast as the number of beams. In other words, the efficiency of multibeam system implemented as a set of separate channels measured by the above mentioned ratios decreases, in comparison with a single-beam system, at least in proportion to a number of beams. This is the main technical and economical problem related to creation of multibeam free-space communication systems designed to provide information exchange for multiple objects (subscribers).
The optical space communication device is known, consisting of at least one receiver and one transmitter implemented as several sources of light located in the same plane around the receiver. The sources are installed with different inclinations to the setting plane so that the angles of deviation from the perpendicular to the plane increases with the distance to the receiver, providing the absence of intersection of beams from sources with different inclinations to the perpendicular (see the description of the Japanese patent application No. 63326970, publication No. H 04 B 10/10/1990 /1/). The disadvantages of the known device are: the difficulty of alignment of the sources relative to the receivers at which they shall be aimed, the difficulty of readjustment of the sources after the receivers have been moved, and large divergence of radiation illuminated by the sources, which requires either high power of the radiation, or high sensitivity of the receivers (both causing an increase of weight and size). The last circumstance (high divergence of the radiation) also requires narrowing the receiving bandwidth, as less power approaching a receiver provide for narrower telecommunication bandwidth for the same reliability of telecommunication.
A telemetry system is known, using an optical communication device (see the description of the Great Britain patent No. 2180116, H 04 B 9/00, 1987 /2/). The communication device is implemented in the form of several receivers and a transmitter consisting of several radiation sources installed on a curved surface, e.g. a hemisphere. The disadvantages of the known device are the following: difficulty of sources alignment relative to receivers corresponding to them and limited area of the system applications due to short ranges of communications caused by large energy losses (because of large divergence of optical beams radiated by the sources), and hence narrow bandwidth of the system.
U.S. Pat. No. 5,909,296 discloses an optical communication device having a transmitter with optical radiation modulation means, and one or more optical radiation receivers with light demodulation means. The transmitter is implemented as a set of radiation sources arranged on a convex surface and aimed at the receivers. To decrease irradiated beams divergence, each radiation source has its own microlens. The disadvantage of this device is its complicated design, which requires a separate lens for each transmitter radiation source. This complexity leads to a corresponding increase of the system weight and cost. Furthermore, the sources of optical radiation are fixed relative to the lenses, which prohibits any correction of the beamwidth and beam direction when the distance and/or angle between the receiver and the transmitter change. Thus, in spite of the irradiated beams divergence reduction due to the lenses, the system efficiency remains low because of excess energy losses as well as because of increased system weight and cost.
U.S. Pat. No. 3,713,163 discloses an antenna capable of delivering a plurality of beams centered about a single boresight axis that includes a focusing means, such as a parabolic reflector or lens. Plural arrays, each including plural radially aligned antenna elements extending along the boresight axis, are located in proximity to a focal point for the focusing means. The elements of the plural arrays are independently exited. Each of the arrays is independently rotated about the boresight axis. The disadvantage of this device is that plural beams derived by it are being aligned about a single boresignt axis and may provide tracking and communication only with targets located within narrow angle. To transmit electromagnetic radiation toward widely separated locations would require multiple instances of the narrow-beam antenna system. Another disadvantage is that antenna elements are located in the focal surface of the focusing means without using mounts enabling their movement along axes of respective radiated electromagnetic beams. This prohibits optimization of beam diameter on a target and causes significant energy loss.
From the above discussion it becomes clear that there is an ongoing need for multi-beam wide-angle optical transmitter and receiver systems, for use in free-space optical communications, which have an increased efficiency. There is a specific need for such systems designed to minimize energy loss and thereby allow increased transmission rates and/or distances. There also is a need for such optical transmitter and receiver systems having a simplified structure, to reduce complexity, weight and cost. There is a further need for such systems that are easily adjustable in the field, to compensate for changes in alignment with remote transceiver units.
The optical communication device, which is the subject of this application, pursues simplification of the design and operation, as well as reduction of energy losses. Also, the proposed free-space optical communication device allows an increase in information transfer bit rate and/or a maximum distance of reliable information transmission relative to weight, cost, and power consumption of a multibeam free-space optical communication system. A further objective of the invention is to reduce the weight and power consumption of the system, and the cost of a unit bandwidth allocated to every subscriber.
To achieve the above stated objectives and overcome the problems with existing systems, the proposed optical communication device includes a transmitter, implemented as several optical radiation sources installed on a curved surface and radiation modulation means. The device is equipped with a single wide-angle objective lens positioned between the radiation sources and respective remote subscriber receivers, in such a manner that the radiation sources are located in the areas optically conjugated with the subscriber receivers.
Preferably, every radiation source is installed on the curved surface in a way providing for its movement along the surface as well as along an optical beam irradiated by the source. Also, to achieve the above objectives, optical receivers may be positioned on the curved surface, so that they are optically conjugated relative to the wide-angle objective with respective remote subscriber radiation sources. Preferably, the receivers are installed on the curved surface in a way providing for their movement along the surface as well as along the optical axes of their received beams. The objective lens may be implemented to allow rotation thereof around an arbitrary axis together with the radiation sources and the receivers.
The distinctive features of the preferred embodiment are:
equipping the transmitter with a single wide-angle objective lens and placing the objective between the radiation sources and the subscriber receivers;
positioning the radiation sources in the areas optically conjugated with the subscriber receivers;
installation of the radiation sources on a curved surface in a way providing for their movement along the surface as well as along the axes of beams irradiated by the sources;
placing the wide-angle objective lens between the receivers and the subscriber radiation sources;
positioning the receivers on the curved surface and their optical conjugation with the subscriber radiation sources relative to the wide-angle objective lens;
installation of the receivers on the curved surface in a way providing for their movement along the surface and along the axes of the received beams;
implementation of the objective lens in a way providing for its rotation around an arbitrary axis together with the radiation sources and the receivers.
Placing of a single wide-angle objective lens between the radiation sources and the remote subscriber receivers makes it possible to simplify the optical communication device design, and hence to increase the multibeam communication system efficiency, because, instead of a multitude of lenses or other optical radiation concentrating elements used for every radiation source and every receiver or for every group of receivers located within narrow angle, only one shared wide-angle objective lens is used.
Positioning the radiation sources in areas optically conjugated with the corresponding subscriber receivers provides for optimal concentration of the radiation from each radiation source positioned on the curved surface onto a photosensitive area of the corresponding subscriber receiver. Thus a further increase of the device efficiency is obtained, which can be measured by an increase of the information transmission bit rate and the range of the system operation, or by a decrease of power consumption and size/weight of the system.
The installation of the radiation sources with an ability to move them along the curved surface provides for reliable device operation via correct positioning of the sources relative to the objective lens and the corresponding subscriber receivers. The possibility of the source shifting within the curved surface and along the beam axis provides for maintenance of optimum optical conjugation of the source and the subscriber receiver if variation occurs in the direction at the subscriber receiver or in the distance between the source and the subscriber receiver, and thus provides for minimization of energy losses and, as a final result, for information transmission rate and range of the device operation increase.
The source shifting along the beam axis also allows to maintain optimal beam diameter on the subscriber optical receiving aperture, providing for a balance between energy concentration onto a photosensitive area of a receiver and the system tolerance to the beam""s axis instability. The larger the beam diameter on a subscriber""s receiving aperture the lower the energy concentration and the higher the tolerance.
The positioning on the curved surface of the receivers optically conjugated with the subscriber radiation sources allows for implementation of a two-way communication with the subscribers using one and the same objective for the transmission as well as for the reception of information, thus further simplifying the device"" design and reducing its cost.
Implementation of the objective lens with an ability of rotating it around an arbitrary axis together with the radiation sources and the receivers simplifies the device"" operation and provides for increase of information transmission rate or operation range of the communication system in specific implementation casesxe2x80x94when the transmitter""s and the subscribers"" receivers and radiation sources are placed on carriers moving relative to each other, e.g. if the transmitter is installed on a high mast swinging under the wind.
Installation of the receivers on the curved surface with an ability to move them along the surface as well as along the axis of the received beams makes it possible to maintain the optical conjugation of the subscriber radiation sources with the receivers, corresponding to them, when angle and distance between the receivers and the subscriber radiation sources changes, and hence to reduce the power consumption and to increase the range of the device operation and information transmission capacity.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.